U.S. patent application number 10/529786 was filed with the patent office on 2006-07-06 for method and arrangement for generating an atmospheric pressure glow discharg plasma (apg).
This patent application is currently assigned to Fuji Photo Film B. V.. Invention is credited to Eugen Aldea, Hindrik Willem De Vries, Fuyuhiko Mori, Mauritius, Cornelius, Maria Van De Sanden.
Application Number | 20060147648 10/529786 |
Document ID | / |
Family ID | 31970433 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060147648 |
Kind Code |
A1 |
De Vries; Hindrik Willem ;
et al. |
July 6, 2006 |
Method and arrangement for generating an atmospheric pressure glow
discharg plasma (apg)
Abstract
Method and arrangement (1) for generating an atmospheric
pressure glow plasma APG (7), where in a plurality of electrodes,
(4, 5) are arranged defining a discharge space (10) for forming
said plasma (7). The electrodes (4,5) are connected to a power
supply (8) providing an AC-voltage having a frequency of at least
50 kHz to the electrodes (4,5). A gaseous substance (6) is provided
in said discharge space and comprises t least one of a group of
argon, nitrogen and air.
Inventors: |
De Vries; Hindrik Willem;
(Tilburg, NL) ; Mori; Fuyuhiko; (Shizuoka, JP)
; Aldea; Eugen; (Eindhoven, NL) ; Van De Sanden;
Mauritius, Cornelius, Maria; (Tilburg, NL) |
Correspondence
Address: |
DAVIDSON BERQUIST JACKSON & GOWDEY LLP
4300 WILSON BLVD., 7TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
Fuji Photo Film B. V.
Tilburg
NL
|
Family ID: |
31970433 |
Appl. No.: |
10/529786 |
Filed: |
September 30, 2003 |
PCT Filed: |
September 30, 2003 |
PCT NO: |
PCT/EP03/10923 |
371 Date: |
November 3, 2005 |
Current U.S.
Class: |
427/569 ;
118/718; 118/723E |
Current CPC
Class: |
H01J 37/32825 20130101;
B05D 2252/02 20130101; H01J 37/32036 20130101; H05H 1/2431
20210501; C23C 16/503 20130101; H05H 1/2406 20130101; B05D 1/62
20130101; B05D 3/141 20130101; H01J 37/32082 20130101; C23C 16/505
20130101 |
Class at
Publication: |
427/569 ;
118/723.00E; 118/718 |
International
Class: |
H05H 1/24 20060101
H05H001/24; C23C 16/00 20060101 C23C016/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2002 |
EP |
02079074.7 |
Claims
1. A method for generating an atmospheric pressure glow discharge
plasma (APG), wherein a plurality of electrodes are arranged
defining a discharge space for forming said plasma, wherein said
electrodes are connected to a power supply and an AC-voltage is
applied to said electrodes, and wherein a gaseous substance is
provided in said discharge space, wherein said AC-voltage applied
to said electrodes has an amplitude equal to at least the breakdown
voltage of said gaseous substance and has a frequency of at least
50 kHz, and said gaseous substance essentially comprises at least
one of a group comprising argon, nitrogen and air.
2. The method according to claim 1, wherein said AC-voltage
amplitude is less than or equal to approximately 140% of said
breakdown voltage.
3. The method according to claim 2, wherein said AC-voltage
amplitude is between 110% and 120% of said breakdown voltage.
4. The method according to claim 1, wherein the temperature of said
gaseous substance is lower than 100 C.
5. The method according to claim 1, wherein at least one further
gas is provided to said gaseous substance in said discharge
space.
6. The method according to claim 5, comprising at least the steps
of: providing said further gas to said discharge space after
essentially stabilising said plasma such that the concentration of
said further gas is fractionally increased stepwise; and
stabilizing said plasma by adjusting said AC-voltage after each
stepwise increment of said concentration of said further gas.
7. The method according to claim 1, wherein said at least one
further gas is provided to said gaseous substance in a
concentration of at most 50% by volume.
8. The method according to claim 7, wherein said concentration is
at most 20% by volume.
9. The method according to claim 5, wherein said at least one
further gas provided to said gaseous substance in said discharge
space is comprised of at least one of a group of O.sub.2, CO.sub.2,
NH.sub.3, common precursor gasses such as SiH.sub.4, hydrocarbons,
organosilicons such as TEOS and HMDSO, or organometallics and
combinations thereof.
10. The method according to claim 9, wherein said gaseous substance
provided in said discharge space is flowed through said discharge
space, establishing a gas flow.
11. The method according to claim 10, wherein said gas flow has a
flow rate in a range of 1 l/min to 50 l/min.
12. The method according to claim 10, wherein the velocity of the
gas flow is in the range of 0.1-10 m/s.
13. The method according to claim 12, wherein the velocity of the
gas flow is in the range of 1-5 m/s.
14. The method according to claim 1, wherein said AC-voltage is
chosen to comprise a frequency less than 1 MHz.
15. The method according to claim 14, wherein said frequency of the
AC-voltage is chosen within a range of 100 kHz to 700 kHz.
16. The method according to claim 1, wherein a residence time for
treating a thermoplastic polymer film in said discharge space is
chosen such that said thermoplastic polymer film is kept at a
temperature below said glass transition temperature of sax a
thermoplastic polymer film.
17. The method according to claim 16, wherein said residence time
is controlled by moving said film through said thermoplastic
polymer discharge space while controlling the velocity of said
thermoplastic polymer film.
18. The method according to claim 16, wherein the amplitude of said
AC-voltage is chosen such that the temperature of the discharge
space remains below a glass transition temperature of said
thermoplastic polymer film during treatment of said thermoplastic
polymer film and for maintaining said glow plasma.
19. The method according to claim 16, wherein said thermoplastic
polymer film comprises at least one of a group comprising triacetyl
cellulose (TAC), polyethyleneterephthalate (PET),
polyethylene-naphthalate (PEN) and similar thermoplastic
polymers.
20. The method according to claim 1 wherein at least one of said
electrodes is covered with a film of dielectric material.
21. The method according to claim 20, wherein said film of
dielectric material is chosen comprising a thickness in a range of
1 .mu.m to 1000 .mu.m.
22. The method according to claim 21, wherein said thickness lies
within a range of 250 .mu.m to 500 .mu.m.
23. The method according to claim 1, wherein at least two of said
electrodes are spaced apart from each other over a distance within
a range of 100 .mu.m to 5000 .mu.m.
24. The method according to claim 23, wherein said distance is
chosen within a range of 250 .mu.m to 1500 .mu.m.
25. The method according to claim 1, wherein a voltage rise time
defines a shortest time interval for said AC-voltage to reach its
maximum value starting from zero, and wherein said voltage rise
time of the AC-voltage is in the range of 0.1 to 10 kV/.mu.s.
26. The method according to claim 1, wherein current density
through said plasma is less than 10 mA/cm.sup.2.
27. The method according to claim 1, used for treating a substrate
in said discharge space with a chemical vapour deposition process
using said plasma.
28. An arrangement for generating an atmospheric pressure glow
discharge plasma (APG), comprising a plurality of electrodes
arranged such that a discharge space is defined by said electrodes,
further comprising means for applying an AC-voltage to said
electrodes, and means for providing a gaseous substance to said
discharge space, wherein said means for applying an AC-voltage to
said electrodes are arranged for applying an AC-voltage having an
amplitude equal to at least a breakdown voltage of said gaseous
substance and having a frequency of at least 50 kHz, and said means
for providing a gaseous substance to said discharge space are
arranged for essentially providing at least one of a group
comprising argon, nitrogen and air having a temperature lower than
100.degree. C.
29. The arrangement according to claim 28, wherein said means for
applying an AC-voltage are arranged for providing an AC-voltage
having amplitude up to 140% of said breakdown voltage.
30. The arrangement according to claim 28, wherein said means for
providing a gaseous substance are arranged for providing at least
one further gas to said gaseous substance in said discharge
space.
31. The arrangement according to claim 30, wherein said means for
providing a gaseous substance are further arranged for providing
the at least one further gas such that the concentration of said at
least one further gas is stepwise adjustable.
32. The arrangement according to claim 30, wherein said at least
one further gas comprises one of a group of 0.sub.2, CO.sub.2,
NH.sub.3, common precursor gasses such as SiH.sub.4, hydrocarbons,
organosilicons such as TEOS and HMDSO, or organo-metallics, and
combinations thereof.
33. The arrangement according to claim 28, comprising means for
flowing said gaseous substance through said discharge space.
34. The arrangement according to claim 32, wherein said means for
flowing said gaseous substance through said discharge space is
arranged for establishing a flow with a flow rate within a range of
1 l/min to 50 l/min.
35. The arrangement according to claim 34, wherein said means for
flowing said gaseous substance through said discharge space is
arranged for establishing a flow with a flow velocity within a
range of 0.1-10 m/s.
36. The arrangement according to claim 28, wherein said means for
applying a high frequency AC-voltage is arranged for applying a
voltage comprising a frequency within a range of 50 kHz to 1
MHz.
37. The arrangement according to claim 28, wherein at least one of
said electrodes is arranged for supporting a thermoplastic polymer
film to be treated by said plasma.
38. The arrangement according to claim 37, further comprising means
arranged for moving said thermoplastic polymer film through said
discharge space with a velocity for which the residence time of
said thermoplastic polymer film is such that the thermoplastic
polymer film is kept at a temperature below a glass transition
temperature of said thermoplastic polymer film.
39. The arrangement according to claim 37, wherein said means for
applying an AC-voltage are arranged for providing an AC-voltage
having an amplitude such that the temperature of the discharge
space remains below a glass transition temperature of said
thermoplastic polymer film during treatment of said thermoplastic
polymer film.
40. The arrangement according to claim 28, including a film of
dielectric material that is contiguous to at least one of said
electrodes.
41. The arrangement according to claim 40, wherein said film of
dielectric material comprises a thickness in a range of 1 .mu.m to
1000 .mu.m.
42. The arrangement according to claim 28, wherein said discharge
space comprises distance between said electrodes within a range of
0.1 mm to 5 mm.
43. The arrangement according to claim 28, wherein the shortest
time interval for said AC-voltage to reach its maximum value
starting from zero, is performed at least in a range of 0.1 to 10
kV/.mu.s.
44. The arrangement according to claim 28, wherein the current
density through said plasma is adjustable in a range below 10
mA/cm.sup.2.
45. The arrangement according to claim 28, further including a
current choke coil arranged for stabilising said plasma.
46. The arrangement according to claim 28, wherein a chemical
vapour deposition treatment process is performed on a substrate in
said discharge space using said plasma.
47. The method according to claim 7, wherein said at least one
further gas provided to said gaseous substance in said discharge
space is comprised of at least one of a group of O.sub.2, C0.sub.2,
NH.sub.3, common precursor gasses such as SiH.sub.4, hydrocarbons,
organosilicons such as TEOS and HMDSO, or organo-metallics and
combinations thereof.
48. The arrangement according claim 30, wherein said at least one
further gas comprises one of a group of 0.sub.2, CO.sub.2,
NH.sub.3, common precursor gasses such as SiH.sub.4, hydrocarbons,
organosilicons such as TEOS and HMDSO, or organo-metallics, and
combinations thereof.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to a method and
apparatus for generating atmospheric pressure glow discharge (APG)
plasma and the use thereof. In particular it is related to a method
and apparatus for generating an atmospheric glow discharge plasma
(APG), wherein a plurality of electrodes are arranged defining a
discharge space for forming said plasma.
BACKGROUND OF THE INVENTION
[0002] Modification or treatment of a surface by applying glow
plasma is a known technique in industries, such as photo film
production industry, used in order to improve certain surface and
material properties. For instance. In the production of photo film,
a thermoplastic polymer film (triacetyl cellulose (TAC),
polyethyleneterephthalate (PET), polyethylene-naphthalate (PEN) or
similar) is prepared using a glow plasma in order to improve
adhesion properties of the surface.
[0003] Plasma is considered generally as a suitable solution for
material processing, because it generates a large flux of reactive
species (radicals, ions), which can be directed to the process zone
and manipulated to the desired shape by using an appropriate
electric field distribution. Plasma treatment would have
considerable advantage if it could be generated at atmospheric
pressure. Advantages of using atmospheric pressure are a larger
density of reactive species than in the low pressure case, and the
advantage of avoiding vacuum technology.
[0004] Another desired feature of atmospheric pressure glow plasmas
(APG) is the generation of these plasmas at low temperatures around
300-400 K. This will make the technology applicable to the
treatment of thermoplastic polymer surfaces, as is common in photo
film production methods.
[0005] Generating a plasma under the above circumstances is not a
straightforward technique. At atmospheric pressure, the particle
density is high and as a result the mean free path of reactive
species is small. The processes of excitation and ionisation are
restricted to a limited area, and the plasma is generated primary
in a filamentary form.
[0006] Plasmas at atmospheric pressures are very unstable and will
tend to go into a spark or an arc in short time after the
breakdown. Any random local increase in a current density will tend
to grow rather than to be damped and plasma will be
constricted.
[0007] Generating a plasma requires the supply of sufficient energy
to a gas such that the gas is ionised. Within the plasma,
collisions and interactions between elements of the gas create
chemically or physically active species, such as metastables, ions,
electrons, and others. Recombinations and transitions of excited
elements to their ground state also causes the emission of photons
from the plasma.
[0008] For the plasma to be sustained, sufficient free electrons
should be present in the plasma. One of the solutions for obtaining
a homogenous atmospheric plasma is the generation of a background
pre-ionization, implying that sufficient seed electrons must be
present in the reactor before the plasma breakdown. These seed
electrons can be created as described above, through interactions
within the plasma itself or can be generated as a result of
interactions between the species present in the plasma and the
surface of the electrodes.
[0009] In general an APG plasma is created by applying an
AC-voltage to a plurality of electrodes. A substrate to be treated,
such as a thermoplastic polymer film, may be transported along the
surface of one or more of these electrodes. The frequency of the
AC-voltage may be varied in order to improve the properties of the
plasma.
[0010] The applicability of a plasma for use in material processing
may be evaluated by determining the following parameters: [0011]
the power coupled into the APG plasma calculated from the applied
voltage and the plasma current; [0012] the exposure non-uniformity
parameter, based on the statistical distribution of the exposure
over the surface; [0013] the minimum time of exposure, wherein 99%
of the surface is exposed to a surface plasma energy dose of at
least 1 J/cm.sup.2 (statistical fluctuations of the exposure taken
into account).
[0014] The abovementioned exposure non-uniformity parameter and
minimum time of exposure will be described in more detail
below.
[0015] In a number of situations and applications, such as in
material processing in photo film industry, it will be advantageous
to have the plasma treatment time as short as possible. The plasma
exposure time t is given by: t = L v ( 1 ) ##EQU1## wherein L is
the length of the plasma reactor and v the line speed.
[0016] A short time of exposure allows the use of high line speeds
and of "medium" size of APG reactors (L=10-20 cm). The energy dose
required for reaching the maximum surface energy at the surface of
the material to be treated may as will be appreciated, depend on a
number of factors, amongst which are the properties of the surface
and the material. It is accepted that and atmospheric pressure glow
plasma treatment with an energy dose of 1 J/cm.sup.2 will be
sufficient to reach the maximum surface energy. However in order to
get an acceptable adhesion between a certain type of polymer such
as Polyethylene (PE) and gelatin an energy dose of between 50 and
100 mJ/cm.sup.2 may already be sufficient.
[0017] The minimum time of exposure required for treating a
material is given by: t minim = SE minim P ( 2 ) ##EQU2##
[0018] wherein P is the power consumed by plasma and S the active
electrode surface, i.e. the part of electrode surface which is
covered by plasma.
[0019] Since the plasma exposure is a stochastic process, whether
or not a local part of the surface is exposed to the plasma is
stochastic as well. This is represented by a stochastic parameter
with an average value of P/S and with a certain dispersion from
this average value. The minimum time of exposure is therefore
determined by the requirement that with a certitude of 99% all
elements of the surface are exposed to an energy dose of at least 1
J/cm.sup.2. Depending on the plasma uniformity, the energy dose
exposed to the surface may be considerable higher in some local
sites at the surface.
[0020] Taking in account the statistical fluctuations of the
exposure the minimum time of exposure is given by: t minim = SE
minim P .function. ( 1 - 3 .times. .times. .sigma. .times. .times.
D D _ ) ( 3 ) ##EQU3## wherein D is the energy dose received
locally at the surface and .sigma.D the statistical dispersion (the
mean standard deviation from the average value). The statistical
dispersion of the exposure is calculated later in this
document.
[0021] Another quality criterium of the treatment process is the
uniformity of the plasma exposure. In order to establish a high
level of uniformity it is required that fluctuations of the
exposure are present only at microscopic scale (not detectable by
the human eye: of the order of 100 .mu.m or smaller). So with a
certitude of 99%, any element of the surface of the material having
a size of 10.sup.-4 cm.sup.2, must be exposed to an energy dose of
at least 1 J/cm.sup.2.
[0022] It is important to note that for streamer discharges, like a
corona or a silent discharge, it is very difficult to meet the
criteria mentioned above. The cause of it is believed to be the
repulsive forces of streamer space charge and the drop of the
electric field near a streamer plasma. As a result, this type of
discharge can not cover all the material to be treated and gaps of
a few millimetres exist between the streamer discharges.
[0023] For a single pulse the dispersion (average relative
variation) of the exposure over a surface unit of 10.sup.-4
cm.sup.2 is given by: .sigma. .times. .times. D sp D _ sp = .sigma.
.times. .times. ( E pulse ) E pulse S 10 - 4 ( 4 ) ##EQU4## wherein
D.sub.sp is the energy dose per unit of surface per pulse, S is the
electrode surface in cm.sup.2 and E.sub.pulse is the energy
delivered to the plasma by a single pulse. For obtaining equation
(4) it was assumed that the value of the current density on each
element of surface is statistically independent.
[0024] Assuming a Poisson statistical distribution of the exposure,
the relative fluctuations of exposure for the exposure to N pulses
will be: .sigma. .times. .times. D D _ = .sigma. .times. .times. (
E pulse ) E pulse S 10 - 4 * 1 N .times. .times. .sigma. .times.
.times. D D _ = 1 ft .times. .sigma. .times. .times. ( E pulse ) E
pulse S 10 - 4 ( 5 ) ##EQU5##
[0025] herein f is the pulse frequency and N=ft is the number of
plasma pulses received by an element of the surface, during
movement of a material surface (for example a surface of a foil)
through the APG reactor.
[0026] The value of the statistical fluctuation of the energy per
pulse can be determined from the waveforms of the plasma current
and the applied AC-voltage using 16 sample pulses. Hence
.sigma.E.sub.pulse=.sigma.(.intg.I.sub.plasma(t)U(t)dt (6) wherein
I.sub.plasma is the plasma current and the U is the AC-voltage
applied to the electrodes of the APG reactor.
[0027] Taking in account statistical fluctuations of exposure and
equation (2), the minimum time of exposure is determined as
follows: P S .times. .times. t minim .function. ( 1 - 3 .times.
.times. .sigma. .times. .times. D D _ ) = 1 .times. .times. J cm 2
.times. .times. .times. P S [ t minim - 3 .times. t minim f .times.
( .sigma. .times. .times. ( E pulse ) E pulse S 10 - 4 ) ] = 1
.times. .times. J cm 2 .times. .times. .times. t minim = ( F + F 2
+ SE minim P ) 2 ; .times. .times. F = 3 2 .times. 1 f .times. (
.sigma. .function. ( E pulse ) E pulse S 10 - 4 ) ( 7 )
##EQU6##
[0028] Equations (5) and (7) clearly reveal the importance of a
high frequency in order to reduce the time of exposure and to
increase the uniformity. The minimum time of exposure, defined by
equation (7), is strongly dependent on the requirements for the
treatment of PEN, PET and PE for improvement of coating adhesion,
and further depends on the minimum exposure and the size of surface
on which significant non-uniformities are admissible.
[0029] A parameter which reflects the plasma uniformity and which
can be used to compare atmospheric pressure glow (APG) plasmas for
different working conditions and applications, is the variation of
exposure over a surface of 1 cm.sup.2 (after a single pulse
exposure): .delta. .times. .times. D = .sigma. .times. .times. D 1
D _ 1 = .sigma. .times. .times. ( E pulse ) E pulse S 1 ( 8 )
##EQU7## In the following .delta.D (measured as a percentage) will
be named exposure non-uniformity parameter hereinafter, and will be
used as a plasma quality parameter together with the minimum time
of exposure, as defined by equation (7).
[0030] The exposure uniformity may be checked by a toner test,
visualising the charge distribution of charge on the surface.
[0031] In this case the ratio d between the size of the toner
build-up spots and the distance between the toner build-up spots
is: d = S .pi. .sigma. .times. .times. ( E pulse ) E pulse ( 9 )
##EQU8##
[0032] Several attempts have been made in order to generate a
stable atmospheric glow plasma.
[0033] In U.S. Pat. No. 6,299,948 there is disclosed a method for
generating a uniform plasma through a proper combination between
the AC voltage and the frequency in a nitrogen gas. The frequency
range used is 200 Hz to 35 kHz, with a preference frequency below
15 kHz, and an amplitude of the applied voltage in the range
between 5 to 30 kV.
[0034] However, processing of materials in plasma at higher
frequencies is beneficial due to the increased number of discharges
over time. Although the charge generated per pulse and the energy
per pulse are only slightly changing with the frequency, when the
operating frequency is increased the number of pulses per unit of
time increases. Therefore the average power (energy released per
unit of time) increases and the average current (charge released
per unit of time) increases almost proportionally with the
frequency. As a consequence, the material processing is faster than
at lower frequencies and, as will be appreciated, this presents a
clear benefit for the industrial process.
[0035] U.S. Pat. No. 5,414,324 describes an apparatus and method
for generating a uniform atmospheric pressure glow discharge plasma
in a frequency range of 1 to 100 kHz. The frequency ranges used in
this document are based on the model of ion trapping, i.e. where
mobility of the ions is so low that they will be trapped in the
plasma gap whereas the mobility of the electrons is sufficiently
high and can reach the electrodes. A gas is present between the
discharge electrodes, which gas at least comprises one of air,
nitrous oxide or a noble gas such as argon, helium, neon, etc. The
document however suggests that for higher frequencies the plasma
becomes unstable and cannot be sustained anymore.
[0036] The application of helium as reaction gas at high frequency
plasma generation has been performed more often. European patents
EP 0 790 525, EP 0 821 273 and EP 0 699 954 disclose methods and
arrangements for generating a glow plasma at high frequencies.
[0037] EP 0 790 525 discloses the generation of plasmas at 10 kHz
in air and 40 kHz, 450 kHz and 13.56 MHz in He with small amounts
of N.sub.2 and/or 0.sub.2. The gas is purged through holes in the
electrodes of the arrangement used. Measurements of the adhesion
properties of plasma treated PET and PEN reveals that an operating
frequency of 450 kHz is favourable to improve adhesion
effectiveness. Both corona and plasma treated samples are compared
in these documents showing that plasma provides superior adhesion
properties.
[0038] An improved embodiment in EP 0 821 273 describes the
adhesion properties of plasma treated PEN, wherein the plasma is
generated in (combinations of) helium and nitrogen as compared to
air for use as a reaction gas, at frequencies of 10 and 450 kHz.
The arrangement used is a corona-like setup comprised of bare
titanium parts positioned in parallel to a drum. The drum is
covered with a silicone layer and a polymeric film is transported
on the drum through the discharge space. The discharge space is
filled with a gas essentially comprising helium and the polymeric
film is exposed to an atmospheric glow discharge. Some experiments
were performed with a reaction gas essentially comprising N.sub.2
at a frequency of 10 kHz.
[0039] In U.S. Pat. No. 5,585,147 there is disclosed a method for
treating the surface of a glass fabric with an atmospheric pressure
glow discharge plasma for cleaning the surface, improving the
adhesion properties thereof and coating the surface with a
organosilane compound. The glass fabric is treated with a plasma
generated in a carrier gas which is essentially comprised of helium
and/or argon, and which carrier gas is mixed with a reaction gas.
In order to achieve the desired cleaning performance and adhesion
properties, the gas mixture is preheated to temperatures between
100.degree. C. and 600.degree. C., before generating the plasma.
These temperatures however make this plasma unsuitable for surface
treatment of temperature sensitive substrates, such as
thermoplastic polymer films.
[0040] The application of helium however in methods for generating
APG plasmas at high frequency (>50 kHz) of the AC-voltage, is
not favourable. Helium occurs in the atmosphere in a concentration
of approximately 5 ppm (parts per million). Some natural gas
deposits however have been found to contain significant amounts of
helium and therefore most helium is obtained from these national
gas deposits. Some of the deposits contain helium in a
concentration above 0.3% by volume. Most of the helium in the world
comes from natural gas deposits found in Texas, Oklahoma, Kansas
and The Rocky Mountains.
[0041] Helium is extracted from natural gas streams using a low
temperature liquefaction process. This low temperature method
separates crude helium--a mixture of more than 50% helium with
nitrogen and small amounts of other gasses--from the liquefied
portion which consists predominantly of hydrocarbons. Several other
techniques including pressure swing adsorption (PSA) and some
cryogenic processes, are used to refine crude helium into a
concentrated helium product. Helium is an expensive gas due to the
fact that it is rare and difficult to extract. It is mainly for
this reason that the use of helium is preferably avoided in
applications described above.
[0042] Another drawback of the application of helium is that due to
the specific weight of helium (0.17 g/l), helium is very volatile.
For this reason helium gas will always try to escape through the
smallest holes of the arrangement. More countermeasures such as
appropriate sealing and a more complex system for the gas injection
means are required in arrangements using helium as a reaction
gas.
[0043] Lastly, helium with an atomic weight of 4 will release a
relatively small amount of energy in a collision with the surface
to be treated. The use of larger, more heavy particles will
exchange larger amounts of energy and will assist in the physical
treatment of the surface. Therefore, in applications such as
surface activation and etching, helium will not be the most
favourite choice.
[0044] Further to this, heating of the carrier gas in the discharge
space, such as is done in U.S. Pat. No. 5,585,147, is not a
preferred option for treating thermoplastic polymer films, such as
is done in photo film production industry. The higher temperatures
may damage the thermoplastic polymers, for instance due to melting
thereof. On the other hand, it has been experienced that the
temperature automatically rises to some extend due to energy
dissipation.
SUMMARY OF THE INVENTION
[0045] It is an object of the present invention to provide a cost
effective method for generating a stable uniform low-temperature
atmospheric pressure glow discharge plasma at high operating
frequencies of the AC-voltage with outstanding surface treatment
capabilities.
[0046] These and other objects of the invention are achieved by a
method for generating an atmospheric pressure glow discharge plasma
(APG), wherein a plurality of electrodes are arranged defining a
discharge space for forming said plasma, wherein said electrodes
are connected to a power supply and an AC-voltage is applied to
said electrodes, and wherein a gaseous substance is provided in
said discharge space, wherein said AC-voltage applied to said
electrodes has an amplitude equal to at least the breakdown voltage
of said gaseous substance and has a frequency of at least 50 kHz,
and said gaseous substance essentially comprises at least one of a
group comprising argon, nitrogen and air.
[0047] The inventors have discovered that with an AC-voltage of a
frequency of 50 kHz and above, by keeping the applied AC-voltage
amplitude equal to at least the breakdown voltage (wherein the
breakdown voltage is characteristic for each carrier gas used),
dissipation of energy can be reduced substantially. Therefore, the
temperature in the discharge space may be controlled. Another
important finding of keeping the applied AC-voltage at the levels
mentioned above is that the uniformity of the generated glow
discharge plasma, as it is defined in equation (8), has been
significantly improved. A decrease in the minimum exposure time as
defined by equation (7) has also been found. The present invention
provides thus, a low temperature in the discharge space, a short
exposure time and a high level of uniformity of the APG. Hence,
this invention can be applied in various process industries
including those where thermoplastic polymers are involved having a
low glass temperature (T.sub.g), since thermal damage to the
thermoplastic polymer can be prevented. As will be appreciated, the
applied voltage must be sufficiently large in order to maintain a
glow discharge plasma. But it has been found that the amplitude of
the applied voltage should not be too high, at least not higher
than 140% of the breakdown voltage of the applied gaseous
substance.
[0048] In order to further control the temperature in the discharge
space, the gaseous substance is kept at a temperature lower than
100.degree. C. Preferably the gas is kept at room temperature, i.e.
temperature between 20.degree. C. and 30.degree. C.
[0049] In the case of a delicate polymer having a low critical
temperatures (e.g. low the glass transition temperature), we may
further reduce the temperature applied to the polymer by reducing
the residence time of the film in the discharge space. This may be
achieved by moving the film through the discharge space with a
certain velocity and adapting the velocity such that the desired
residence time, for which the temperature of the film is kept such
that the film remains intact, is achieved. Reducing the residence
time (e.g. by adapting the velocity of the film) may be performed
independent of reducing the AC-voltage amplitude as described
above; either of these measures may be performed separately or both
together.
[0050] An increase of the AC-voltage frequency yields an increasing
build up of metastables, interacting with the electrodes and the
plasma to improve plasma stability and density. Increase of the
frequency furthermore, yields build up of the number of active
species contributing to the suppression of filamentary discharge.
This behaviour has been established in argon, air, nitrogen and
helium, however other gasses did not reveal this behaviour.
[0051] Argon, nitrogen and air are comprised of much larger and
heavier atoms than helium (Ar 40, N.sub.2 28, air is a mixture of
roughly 78% N.sub.2, 20% 0.sub.2, 1% Ar and 1% H.sub.20 and some
other fractions) and the use of said gasses is preferred in
applications for surface treatment, in the light of the above
discussion. It will further be appreciated that the use of air,
nitrogen or argon is a more cost effective solution than the use of
helium. For this reason, the invention improves the applicability
of AGP plasmas at high frequencies in a number of industrial
processes.
[0052] Good results have been achieved in particular by providing
an atmosphere rich of argon in the discharge space. It has been
observed that a stable and uniform APG plasma can be achieved this
way.
[0053] In an embodiment of the invention, the AC-voltage is reduced
to levels of at most 140% of the breakdown voltage, after the
plasma has been generated. A preferred range, of the applied
AC-voltage after establishing the plasma is between 110% and 120%
of the breakdown voltage.
[0054] In another embodiment the gaseous substance comprises, in
addition to argon, at least one further gas such as one or more of
a group comprising 0.sub.2, CO.sub.2, NH.sub.3, common precursor
gasses such as SiH.sub.4, hydrocarbons, organosilicons such as TEOS
and HMDSO, or organo-metallics and combinations thereof. As was
observed, in the presence of quantities of these gaseous chemical
substances, a plasma can be sustained more easily. The carrier gas
(comprised of either Ar, N.sub.2, air or combinations thereof) may
comprise up to 50% other gasses by volume, although good results
can be achieved using concentrations of approximately 20% by
volume.
[0055] A preferred embodiment of the present invention comprises at
least the steps of essentially stabilizing the plasma, such that a
homogeneous plasma is achieved in the presence of the carrier gas,
and providing the further gasses mentioned above to the discharge
space such that the concentration of said further gas is
fractionally increased stepwise. Each step of fractionally
increasing, the concentration is followed by a step of stabilizing
the plasma by adjusting the AC-voltage, until a stabile plasma at a
desired concentration is achieved.
[0056] A stepwise fractional increase as mentioned may involve gas
concentration increments of the concentration of further gasses,
which increments are relatively small as compared to the finally
desired concentration. For example, increasing the concentration to
its maximum desired value in for instance 4 steps (of 25% of the
total) or 10 steps (10% of the total) or even 20 steps (5% of the
maximum desired concentration) are regarded as relatively small
steps as compared to the finally desired concentration.
[0057] Defining the voltage rise time as the shortest time interval
needed for the sine voltage to raise/decrease from zero voltage
amplitude to maximum/minimum voltage amplitude, stabilising the
plasma may be achieved by adjusting the voltage characteristics
such that the voltage rise time is in the range of 0.1 to 10
kV/.mu.s. An optimum is reached with values in the range of 0.1 to
5 kV/.mu.s.
[0058] It has also been observed that a stabile APG plasma can be
easily achieved by keeping the plasma current density below 10
mA/cm.sup.2.
[0059] The effects and benefits mentioned above may be enhanced in
an embodiment where the discharge space is flowed through by said
gaseous substance.
[0060] An embodiment of the present invention, directed to the
treatment of a substrate film by a plasma may be achieved when at
least one of the electrodes supports the substrate film to be
treated. Said substrate film may be transported through the
discharge space over the surface of the at least one electrode,
exposing the substrate to the AGP plasma discharges.
[0061] In yet another embodiment of the present invention at least
one of the electrodes is covered with a dielectric material. It has
been observed that the surface of the dielectric plays a decisive
role in the stability and performance of the plasma.
[0062] Above-mentioned dielectric material is not necessarily of
the same nature as the substrate film to be treated. The dielectric
material may be coated on the surface of at least one of the
electrodes in order to improve the behaviour of the APG-plasma.
[0063] It has further been observed that a method provided by the
invention can be used under the circumstances (with reference to
the operation parameters) presented in the table below:
TABLE-US-00001 Parameter Description Range Optimum U AC-voltage
100%-140% of 110%-120% of amplitude breakdown the breakdown
voltage. voltage f AC-voltage 50 kHz-1 MHz 100-700 kHz frequency
.phi..sub.vol, gas volumetric gas 1-50 l/min 10 l/min flux of gas
flow flow velocity 0.1-10 m/s 1-5 m/s d.sub.gap gap distance
100-5000 .mu.m 250-1500 .mu.m between electrodes on opposite sides
of the discharge space d.sub..epsilon. thickness of 1-1000 .mu.m
250-500 .mu.m dielectric material on at least one electrode
concentration volumetric <50% approximately further gasses
percentage 20% present in carrier gas Voltage rise shortest time
0.1-10 kV/.mu.s 0.1-5 kV/.mu.s time from zero to maximum/minimum
voltage Plasma current 0-10 mA/cm.sup.2 .ltoreq.5 mA/cm.sup.2
density Note that for carrier gasses such as argon, nitrogen and
air, the desired voltages are in the range of 1-6 kV.
[0064] In another aspect of the invention there is provided an
arrangement for generating an atmospheric pressure glow discharge
plasma (APG), comprising a plurality of electrodes arranged such
that a discharge space is defined by said electrodes, further
comprising means for applying an AC-voltage having a frequency of
at least 50 kHz to said electrodes, and means for providing a
gaseous substance to said discharge space, characterised in that
said means for providing a gaseous substance to said discharge
space are arranged for essentially providing at least one of a
group comprising argon, nitrogen and air.
[0065] The above mentioned invention may be applied in various
processes in industry, such as in surface activation processes
wherein substrate can be glass, polymer, metal, etcetera (specific
examples of this are modification of surface properties such as
improving adhesion or creating hydrophobic or hydrophilic
properties); in the chemical vapour deposition process where
specific chemical compositions gasses such as SiH4, hydrocarbons,
organosilicons (TEOS, HMDSO, etc.) or organo-metallics are usually
involved; in deposition processes of polymers and in deposition
processes for oxidic materials; in the surface cleaning processes
of various substrates where sterilisation or dry cleaning purposes
can be realised. Furthermore the invention may also be excellently
applied below atmospheric pressures, such as between 100 mbar and 1
bar (atmospheric pressure).
[0066] The present invention will now be further elucidated by
description and drawings referring to a preferred embodiment
thereof, directed to the treatment of a substrate service with an
APG. The invention is not limited to the embodiments disclosed,
which are provided for explanatory purposes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0067] FIG. 1 is a schematic drawing of an arrangement for carrying
out a method according to the present invention;
[0068] FIG. 2 is a drawing of an APG-plasma treatment device for
treating substrate films;
[0069] FIG. 3 shows a diagram of the dependency of plasma quality
parameters in a plasma generated according to the present invention
using Ar as the carrier gas;
[0070] FIG. 4 shows a diagram of the dependency of plasma quality
parameters in a plasma generated according to the present invention
using N.sub.2 as the carrier gas;
[0071] FIG. 5 shows a diagram of the dependency of plasma quality
parameters in a plasma generated according to the present invention
using air as the carrier gas;
[0072] FIG. 6 is a characteristic of the applied voltage and the
plasma current in a plasma generated using a method or arrangement
according to the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0073] In FIG. 1 an arrangement for generating plasma 1 comprises a
dielectric 2 connected to a first electrode 4. A second electrode
5, arranged for carrying a substrate film 3, is arranged on the
opposite side of a discharge space 10. A gas flow 6 may be
established parallel to the surfaces of said dielectric 2 and
substrate film 3. First electrode 4 has been connected to an
adjustable AC power supply 8, arranged for providing an AC-voltage
with a frequency larger than 50 kHz. Second electrode 5 has been
connected to said AC power supply 8 as well as to ground 9.
[0074] In a method according to the present invention, a gas flow 6
is established comprising at least one of a group of argon, air and
nitrogen. For example, pure argon may be used as a carrier gas, or
alternatively a mixture of argon and air or nitrogen and argon may
be used in arbitrary concentrations. The examples described
hereinafter have been carried out with nitrogen, air and argon.
Surprisingly, it has been observed that with a method according to
the present invention, the presence of argon, air or nitrogen as a
carrier gas has a positive influence on the plasma stability and
intensity.
[0075] After establishing a gas flow, the electrodes 4, 5 are
energised by the adjustable AC power supply 8. Power supply 8 has
been adjusted such that an AC voltage with an amplitude in the
range of 3-6 kV, at most 40% higher than the breakdown voltage
(optimally the amplitude may be between 10% and 20% higher than the
breakdown voltage), at a frequency in the range of 50 kHz-1 MHz
(optimum 100-700 kHz) is provided. Upon reaching a breakdown
voltage, a plasma 7 will be generated between the dielectric 2 and
the substrate film 3. Simultaneously, the substrate film 3 may be
moved in any direction past the plasma in order to treat said film
3 along its full surface.
[0076] The carrier gas may further comprise oxygen, carbon dioxide,
NH.sub.3, common precursor gasses such as SiH.sub.4, hydrocarbons,
organosilicons such as TEOS and HMDSO, or organo-metallics or any
combination or mixture of these gasses. These gasses may be present
in the carrier gas from the beginning of the process. Good results
have been achieved by adding the further gasses after a step of
stabilising the plasma. This adding can be done best in fractional
stepwise increments and by stabilising the plasma after each step
until a desired concentration is established (for instance
20%).
[0077] A stepwise fractional increase as mentioned may involve gas
concentration increments of the concentration of further gasses,
which increments are relatively small as compared to the finally
desired concentration. For example, increasing the concentration to
its maximum desired value in for instance 4 steps (of 25% of the
total) or 10 steps (10% of the total) or even 20 steps (5% of the
maximum desired concentration) are regarded as relatively small
steps as compared to the finally desired concentration.
[0078] Alternatively, the addition of chemically reactive gases to
the carrier gas is done after a homogenous glow plasma was ignited
in the carrier gas and operated for approximately 10-20 s. The
concentration of the reactive gas added to the carrier gas is
gradually increased in steps of a few percent of the concentration
followed by a gradual increase of the voltage applied to the plasma
with about 5-10% per step in order to maintain the plasma
stable.
[0079] The dielectric 2 may comprise a material such as PET, PEN,
PTFE or a ceramic such as silica or alumina. Any combination or
mixture may be used as well. In photo film production industry, the
substrate film 3 may be a support for photo sensitive layers, which
is treated with an APG plasma first, in order to improve its
adhesion properties before adding said photo sensitive layers. Said
support may be any suitable polymer, but often materials such as
PEN, PET, TAC, PE, or Polyolefin laminated paper will be used for
this purpose.
[0080] Stabilising the plasma may be achieved by adjusting the
voltage characteristics such that the voltage rise time is in the
range of 0.1 to 10 kV/.mu.s. An optimum is reached with values in
the range of 0.1 to 5 kV/.mu.s. The voltage rise time is dependent
on the frequency and the amplitude of the AC-voltage and is
influenced by the dielectric permeability of the dielectric 2 and
the thickness of the dielectric material.
[0081] It has also been observed that a stabile APG plasma can be
easily achieved by keeping the plasma current density below 10
mA/cm.sup.2, especially at plasma current density equal to or lower
than 5 mA/cm.sup.2.
[0082] The distance between the dielectric 2 and the substrate film
3, forming the discharge space 10, may be in the range of 0.1-5 mm,
with an optimum distance of 250-1500 .mu.m. The volumetric gas flux
of gas flow 6 may be within the range of 1 l/min and 50 l/min, with
an optimum around 10 l/min. The thickness of dielectric 2 may be in
a range of 1-1000 .mu.m, with an optimum between 250 .mu.m and 500
.mu.m.
[0083] FIG. 2 shows a device according to the present invention for
treating a substrate film 3, such as a photo film. In this device
electrodes 4 form the inside of a half circle around roll shaped
drum electrode 5. The electrodes 4 are attached to an electrode
holder 11. The drum shaped electrode 5 is a roll supporting a
substrate film 3, which film 3 can be moved through a discharge
space 10, formed by said electrodes 4 and 5, by a rotating motion
of drum electrode 5. A dielectric coating 2 may be comprised on the
electrodes 4 or, underneath the supported film 3, electrode 5. Note
that only a few electrodes 4 are shown in FIG. 2. The electrodes 4
are connected to an output of adjustable power supply 8, while the
other output of power supply 8 and drum electrode 5 by connection
36 are connected to ground 9.
[0084] Via support roll 13 the substrate film 3 is brought inside
treatment chamber 20, and is lead over drum electrode 5 via
supporting roll 12. After treatment, substrate film 3 will leave
the treatment chamber 20 via supporting roll 14. In use, a plasma
will be generated in discharge space 10 by energizing said
electrodes with adjustable power supply 8. The surface of substrate
film 3 is treated by said plasma as it is moved through said
discharge space 10.
[0085] The treatment chamber further comprises a gas inlet 15, a
gas outlet 16, and means 17 for establishing a gas flow 6 in the
discharge space 10. A flexible wall 18 between inlet 15 and support
roll 12 prevents the forming of a direct gas flow between inlet 15
and outlet 16. As a result, a gas flow 6 is forced to flow through
discharge space 10.
[0086] Gas flow 6 essentially comprises at least one of a group of
argon, air, nitrogen or combinations thereof. Additionally further
gasses may be added to the gas flow 6, such as oxygen, carbon
dioxide, NH.sub.3, common precursors used in chemical vapour
deposition processes or any combination of these gasses. Preferably
the amount of other gasses present in the carrier gas does not
exceed 50% by volume. One or more gas storage means 22 comprising
argon, air or nitrogen (as an example a gas bottle comprising argon
is shown in FIG. 2) is therefore connected to the gas inlet 15, via
adjustable valve 34 and element 21 combining a plurality of gas
flows as shown. Said further gasses, such as oxygen, carbon
dioxide, NH.sub.3, common precursors, etcetera, mentioned above may
be stored in a plurality of storage means 23 and added by element
24. Element 24, adding the further gasses used is connected to
element 21 via adjustable valve 19, which valve 19 may be stepwise
adjustable such that the concentration of further gasses can be
increased or decreased stepwise. It will be appreciated that a
continuously adjustable valve enables stepwise adjustment of the
concentration as well. The use of a stepwise adjustable valve 19 is
therefore preferred, but remains optional.
[0087] Stepwise adjustable valve enables a method as described
herein-above, wherein the adding of further gasses is performed in
fractional stepwise increments and by stabilising the plasma after
each step until a desired concentration of further gasses is
established. It will further be appreciated that the requirements
for the valve 19 are determined by the magnitude of the fractional
steps desired (some examples are given above).
[0088] One or more current choke coils (not shown) may be added to
the arrangement in order to control the voltage during plasma
breakdown.
[0089] The invention will further be described by describing a
number of embodiments thereof, which embodiments have (amongst
other embodiments) been tested in a laboratory environment.
EXAMPLE 1
[0090] In a first example, an atmospheric pressure glow discharge
plasma was generated by applying argon gas at room temperature with
a velocity of 2 m/s into the discharge space at an amplitude of the
applied voltage of 1.4 kV. Both electrodes are covered with a
dielectric material comprising of polyethylene-naphtalate and
having a thickness of 100 micrometers. The gap distance between the
electrodes (typically forming the discharge space) is 1 mm. An LC
matching network is incorporated in the plasma arrangement. The
results of the measurements are listed in the table below.
TABLE-US-00002 f [kHz] P [W] .delta.D [%] t.sub.minim [s] 1 15 28
89 129 20 14 59 29 28 10 41 70 50 5 2 300 120 2 9
[0091] FIG. 3 shows a diagram of the dependency of plasma quality
parameters, as defined earlier in this document, on the frequency.
The diagram shows the power P coupled into the APG plasma 25, the
exposure non-uniformity parameter .delta.D 26 and the minimum time
of exposure t.sub.minis 27. The parameters are based on
measurements of the AC-voltage applied and the plasma current and
are the results of a comparative study of the properties of an
argon (Ar) plasma at low and high frequency of the AC-voltage
applied. The values are plotted on a double logarithmic scale.
[0092] The advantages of generating an APG plasma at high
frequencies (>50 kHz) are supported by the results shown in this
diagram. The power density increases almost a factor 10 whereas the
treatment time and non-uniformity parameter decrease a same order
of magnitude. In addition (not shown in FIG. 3) it was discovered
that large gaps (dimensions of the discharge space) are allowed
without any destabilizing occurring in the plasma.
EXAMPLE 2
[0093] In a second example, an atmospheric pressure glow discharge
plasma was generated by applying nitrogen gas (N.sub.2) with a
velocity of 2 m/s into the discharge space at an amplitude of the
applied voltage of 4.5 kV. Both electrodes are covered with a
dielectric material comprising of polyethylenenaphtalate (PEN) and
having a thickness of 100 .mu.m. The gap distance between the
electrodes (typically forming the discharge space) is 1 mm. An LC
matching network is incorporated in the plasma arrangement. The
results of the measurements are listed in the table below and are
presented in the graph in FIG. 4. Again the power P coupled into
the APG plasma 28, the exposure non-uniformity parameter .delta.D
29 and the minimum time of exposure t.sub.minim 30 are shown.
TABLE-US-00003 f [kHz] P [W] .delta.D [%] t.sub.minim [s] 129 42 39
46 30 45 28 36 70 78 18 2 282 220 7 6
EXAMPLE 3
[0094] In a third example, an atmospheric pressure glow discharge
plasma was generated by applying air with a velocity of 2 m/s into
the discharge space at an amplitude of the applied voltage of 4.5
kV. Both electrodes are covered with a dielectric material
comprising of polyethylenenaphtalate (PEN) and having a thickness
of 100 .mu.m. The gap distance between the electrodes (typically
forming the discharge space) is 1 mm. An LC matching network is
incorporated in the plasma arrangement. The results of the
measurements are listed in the table below and are presented in
FIG. 5. Again the power P coupled into the APG plasma 31, the
exposure non-uniformity parameter .delta.D 32 and the minimum time
of exposure t.sub.minim 33 are shown. TABLE-US-00004 f [kHz] P [W]
.delta.D [%] t.sub.minim [s] 129 46 290 63 30 52 120 135 70 89 70 5
282 290 20 9
EXAMPLE 4
[0095] As an example of a measurement at a frequency of
approximately 280 kHz, FIG. 6 presents a diagram of the AC-voltage
applied to the electrodes and the plasma current of a plasma
generated using a method or arrangement according to the present
invention. In this particular example, the carrier gas used was
argon at a flow velocity of 2 m/s, the gap distance was 2 mm. The
plasma current is shown by the solid line and the applied voltage
by the dotted line.
[0096] The results of the above-mentioned experiments show a stable
APG plasma can be generated in the presence of argon, nitrogen and
air at high frequency. However the operation window for maintaining
a uniform and stable plasma for the nitrogen and air gasses are
rather limited. For argon gas said phenomena is surprisingly not
observed. On the other hand, it is observed that at frequency
ranges between 200 to 500 kHz, the plasma non-uniformity of the
gasses are one order of magnitude smaller than those obtained in
the frequency between 1 and 10 kHz. Said observation shows a clear
benefit for generating APG plasmas at a higher frequencies,
preferably at a frequency of at least 50 kHz. More preferable is a
frequency in the range of 100 to 1 MHz, and most preferable in the
range of 100-700 kHz.
[0097] The increased stability of plasma at high frequency is
believed to be ascribed to the role of metastables providing the
pre-ionization which required for a glow plasma breakdown. Due to
the frequency increase the interval between the discharge pulses
decreases and consequently the density of metastables present in
the discharge space from the previous discharge pulse
increases.
[0098] Although the increase of frequency shows benefits with
respect to the material processing efficiency, it is challenging to
generate an atmospheric glow plasma at frequencies higher than 1
MHz. This can be attributed to the requirement that during the
unstable state of plasma (the plasma breakdown) the voltage applied
to the plasma must decrease as fast as possible. This may be
achieved by covering the electrodes with a dielectric (dielectric
barrier discharge system, DBD). After the breakdown, the electric
field generated by the plasma charge which is accumulating on the
dielectric surface will decrease the voltage applied to the
plasma.
[0099] When the frequency increases, the rise time of the applied
voltage will also increase and also the duration of the
transitional state during which the voltage applied to the plasma
increases before being lowered by the field of the charge
accumulated on the dielectric.
[0100] Additionally, the maximum allowed plasma current with a DBD
system increases proportional with the frequency, so that at higher
frequencies the current can be very high increasing the probability
of the glow to an (unfavourable) arc transition.
[0101] For the purpose of comprehensiveness, it is noted here that
numerous modifications and variations of the present invention are
possible in the light of the above teachings. It is therefore
understood that, within the scope of the amended claims, the
invention may be practised otherwise than as specifically described
herein.
* * * * *